[website] Update selection colors for readability

[Malvoz]

I sometimes find myself selecting parts of text that I'm reading, the current selection styles are not optimal for readability:

PR preview

I've re-used current colors and already-defined custom properties to make selected text more readable:

This could be improved further to ensure a certain level of contrast of course, if anyone is interested in doing that.

[Malvoz]

A somewhat related (to colors) but off topic comment:

CSSWG is looking to potentially standardize color ramps, which may not only be of interest to your organization but perhaps you've got some valuable insight. See https://twitter.com/tabatkins/status/1442979946914205712.